1. Field of the Invention
The invention concerns an electronic radiation imaging system. More specifically the invention concerns an electronic system for imaging by means of X-rays or another type of electromagnetic radiation and its use in certain radiation imaging applications.
2. Description of Related Art
Radiation imaging by means of X-rays has normally used film to record the radiation after, e.g., X-ray exposure of the object of which an image has to be obtained and also in order to provide the imaging directly. In recent years electronic detectors have been employed in order to record the radiation and display the image graphically on a display unit, e.g. a cathode ray tube. The electronic detectors are, e.g., based on photo-conductive or other photo-electric components. The latest technique for radiation imaging has adopted charge-coupled devices.
From U.S. Pat. No. 3,812,361 there is known a device for providing radiation imaging by means of gamma rays. This device is based on a detector with electrode stripes arranged in parallel on the front and back respectively of a sheet of semi-conductor material. The stripes on the front extend in a direction which forms an angle with the stripes on the back. This angle may, e.g., be 90 degrees, thus making the electrode stripes on each of the sides mutually orthogonal. A gamma ray, i.e. a photon, which passes through an electrode on the front and the back respectively will thereby indicate a precise position, viz. an x,y position, which is the point of intersection between the electrode concerned at the front and the back respectively.
The most advanced method for performing electromagnetic radiation imaging electronically is based on charge-coupled components. These form a matrix of detection elements which permit a precise determination of the position of a radiation event. One disadvantage is that charge-coupled components have a dissipation depth, i.e. a sensitivity range, which in todays commercial system is only 15 .mu.m thick. Since X-rays normally have very high energy, they will easily pass through the semi-conductor material, e.g. silicon. The sensitivity of the detector, however, is dependent on the interception of as large a percentage of the X-rays as possible and it will require a much thicker dissipation depth. That part of the radiation which passes through the detector cannot be used to form an image and that means that the radiation dose which is necessary in order to provide an image of a given quality can be lower in a detector with higher sensitivity i.e. thicker dissipation depth. Another disadvantage of charge-coupled components is that they have difficulty in detecting the absorption of individual photons, since the recording of such a detection will be masked by noise during the read-out and thereby be dependent on the energy of the photon. This disadvantage can be mitigated to some extent by having the radiation integrated over time by a detector based on charge-coupled components. However, the integration time is limited, since a charge-coupled component will only be capable of performing such an integration during a limited period of time. If the integration time is too long, the charge will leak to earth and it will also be impossible to perform a reading of a detector based on charge-coupled components at random times.